This invention relates generally to telephony, and more particularly to telephony protection devices.
Telephones are an integral part of global communication. As such, a vast number of telephone lines are connected to private residences and commercial buildings. These lines, made of copper, are extremely conductive, requiring that special consideration be given to hazardous overvoltage and overcurrent conditions and to radiated electromagnetic noise which can interfere with, among others, television and radio. The overvoltage and overcurrent conditions are particularly troublesome since they can cause injury to people and damage to equipment.
These are legitimate concerns due to the hostile environment in which the lines operate. For example, a large number of lines are hung from telephone poles which may be struck by lightning, which can produce hazardous overvoltage and overcurrent in a line connected to a residence or business. Alternatively, the telephone lines are sometimes hung on the same poles as high tension wires. If, for instance, a violent storm or a car crashing into the pole should cause one of the high tension wires to fall onto a phone line, a power cross results, possibly causing overvoltage and overcurrent conditions. Similarly, if someone accidentally cross-wires a phone line to an electrical main, a power cross results. Thus, protection is needed to deal with overvoltage, overcurrent, and radiated electromagnetic interference.
In any telephone subscriber line interface, there are usually two levels of protection. First, the primary protectors deal with the initial overvoltage and overcurrent conditions on the line. These suppression devices are usually either gas tubes or heat coils, and although they take the brunt of the overvoltage and overcurrent, up to 25 kilowatts can still pass through. Thus, secondary protectors are needed to deal with the residual overvoltage and overcurrent conditions, as well as electromagnetic interference. The various embodiments disclosed in the Detailed Description are directed to secondary protection.
Typically, secondary protector circuits are unbounded, having many discrete components and requiring between xc2xc square inch and 1 square inch per channel for every interface termination. This can amount to the protection circuit occupying up to half of the area needed for the interface.
In one embodiment, an apparatus is disclosed which comprises a substrate capable of suppressing electromagnetic fields, with a channel formed therein, a current dependent circuit interrupter disposed inside the channel, and voltage management circuitry coupled to the substrate and electrically coupled to the current dependent circuit interrupter so as to form a crowbar circuit in the presence of overvoltage or undervoltage conditions as determined according to a reference voltage.
In various embodiments, the voltage management circuitry can be directly coupled to the substrate, or the voltage management circuitry can be coupled to a printed circuit board which is coupled to the substrate.
In various embodiments, the substrate is a ferrite material. Among others, the current dependent circuit interrupter can be at least one of a physical link fuse and a self-resettable positive temperature coefficient thermistor. In addition, the voltage management circuitry can be at least one of a symmetrical thyristor arrangement and a zener diode.
The various embodiments described herein advantageously provide for suppression of electromagnetic radiation and protection against overcurrent and overvoltage while reducing the amount of space required for such protection. For example, the various embodiments only require between one sixth to one tenth of the space currently needed by existing secondary protection circuitry.